The question of hair in general has always intrigued me, well, truthfully, it has often grossed me out, quite frankly. I come from a rather large family with one older brother and younger twin sisters. We all shared one bathroom growing up and I was disgusted when I took a shower after my sisters, who have very long hair. The drain was constantly filled with masses of hair that they left behind for me to clean. As you may imagine, there were many screaming matches that ensued. Too Much Information possibly, but cleaning out hair that is more than 12 inches in length is just too much to handle early in the morning. So, why does hair on all other parts of your body stay short, while the hair on your head can grow very long?
I’ll start with a few fun facts about hair provided by the American Academy of Dermatology How Hair Grows are; that the only places hair doesn’t grow on our bodies are the soles of our feet and the palms of our hands; hair you can see is actually dead, which is why you don’t feel pain when you get your haircut; and, we lose anywhere from 50 to 100 strands of hairs a day!
To get to the answer to the question, it is important to first understand the science in hair growth. Every human is born with about 5 million hair follicles, the small cavities from which hair reaches the surface of our skin. Anagen, defined here, is the growth cycle and the first phase of hair growth. It occurs when blood flow enters into the hair follicle and feeds oxygen to stem cells. When these stem cells divide they form keratinocytes, defined here, which are the building blocks for the root of a hair. As keratinocytes continue to divide and push towards the surface of the skin, they die and form keratin, defined here, which is the protein that holds a strand of hair together. This process continues until the strand, which is then dead, pops through the follicle to the surface of your skin. The second phase of hair growth is called Catagen, defined here, or the regression phase, which lasts for up to two weeks. About 3% of our hair is in this phase at all times. In this phase, the blood supply gets cut off at the bottom of the follicle which stops the creation of keratinocytes. This phase stops the growth of hair in that follicle and the follicle shrinks and pushes the strand to the surface. The third phase is called Telogen, defined here, or the resting phase in which hair does not grow any further. About 8% of our hair is in this phase at all times. It is in this phase that the hair that can be seen above the skin falls out. This is the phase in which we lose those 50-100 strands of hair per day.
The answer to the question of why does the hair on all other parts of your body stay short, while the hair on your head can grow very long is in the Anagen, or the first phase of hair growth. Hair grows at a rate of about 1 cm every 28 days. The hair on our heads grows continually for 2 to 6 years in this phase as compared to other parts of our body like our eyebrows, eyelashes, arms and legs, which only grow for up to 45 days. Interestingly, hair on different parts of our bodies grow at different rates per month. For example, eyebrows grow at a rate of only 4.2mm per month.
In conclusion, scientists have figured out the way in which hair grows and the speed in which grows. They are, however, still trying to conclude how the Anagen phase is driven for each body part. Genetics play a role to some degree distinguished by different hair types. Chemical growth signals directed by stem cells in the skin is the most current determination as to how our hair grows. While we might not know exactly how each body part knows not to allow hair to grow too long, we can all be thankful that certain areas don’t grow hair like the hair on our scalps.